Background: Improving unipolar implantable cardioverter-defibrillator (ICD) effectiveness has favorable implications for ICD safety, efficacy, and size. Advances in defibrillation efficacy would accelerate ICD ease of use by decreasing device size and by minimizing morbidity and mortality related to an improved defibrillation safety margin. The specific purpose of the present study was to determine whether unipolar defibrillation efficacy could be improved further in humans by lowering biphasic waveform capacitance.
Methods and results: We prospectively and randomly compared the defibrillation efficacy of a 60-microF and a 120-microF capacitance asymmetrical 65% tilt biphasic waveform using a unipolar defibrillation system in 38 consecutive cardiac arrest survivors before implantation of a presently available standard transvenous defibrillation system. The right ventricular defibrillation electrode had a 5-cm coil located on a 10.5F lead and was used as the anode. The system cathode was the electrically active 108-cm2 surface area shell (or "can") of a prototype titanium alloy pulse generator placed in a left infraclavicular pocket. The defibrillation pulse was derived from either a 60-microF or a 120-microF capacitance and was delivered from RV-->CAN. Defibrillation threshold (DFT) stored energy, delivered energy, leading-edge voltage and current, pulse resistance, and pulse width were measured for both capacitances examined. The 60-microF capacitance biphasic pulse resulted in a stored-energy DFT of 8.5 +/- 4.1 J and a delivered-energy DFT of 8.4 +/- 4.0 J. In 34 of 38 patients (89%), the stored-energy DFT was < 15 J. Leading-edge voltage at the DFT was 517 +/- 128 V. Mean pulse impedance for the 60-microF waveform was 60.6 +/- 7.1 omega. The 120-microF capacitance biphasic pulse resulted in a stored-energy DFT of 10.1 +/- 7.4 J and a delivered-energy DFT of 10.0 +/- 7.2 J (P = .13 and .13, respectively). In 28 of 38 patients (74%), the stored-energy DFT was < 15 J (P = .052). Leading-edge voltage at the DFT with the 120-microF capacitance pulse was 386 +/- 142 (P < .00001). Mean pulse impedance for the 120-microF waveform was 60.7 +/- 7.0 omega (P = .80).
Conclusions: The results of the present study suggest that a relatively small capacitance, 60 microF, can be used for unipolar defibrillation systems without compromising defibrillation energy requirements compared with more typical ICD capacitance values, but this will require a higher circuit voltage. The use of lower capacitance also provides a modest increase in the percent of patients who have very low energy defibrillation requirements, an important issue should maximum ICD energy be decreased from the present level of 34 J. Such a move to smaller output devices could allow significant decreases in device size, a necessary feature of making cardioverter-defibrillator implantation comparable to that of standard pacemaker surgery.